1. Field of the Invention
The present invention relates, in general, to spacer grids used for supporting a plurality of fuel rods in a nuclear fuel assembly and forming a strong turbulent flow of coolant in the assembly and, more particularly, to a spacer grid having double deflected vanes, integrally formed into an upper portion thereof for bending the axial flow of coolant into a swirl flow around the fuel rods placed in square cells, thus more effectively cooling the fuel rods. The vanes are sufficiently wide at their base portions so as to prevent an unexpected bending thereof due to their contact with fuel rods during an insertion of fuel rods into the cells, the vanes also making a smooth variation in the cross-sectional area of the coolant channel at the outlet of the spacer grid, thus preferably reducing pressure loss during an operation of the reactor.
2. Description of the Prior Art
As shown in FIG. 8, a conventional nuclear fuel assembly of a nuclear reactor comprises a plurality of fuel rods 200, each fabricated such that a fissionable fuel material, such as a uranium core, is contained in a hermetically sealed, elongated zircaloy tube, known as the cladding. The fuel rods 200 are placed and supported within the fuel assembly by a plurality of spacer grids 700, which also form a strong turbulent flow of coolant within the fuel assembly. The bottom end plate 300 and top end plate 400 stably connect the fuel rods with the lower and upper structure of the reactor core, respectively.
In the fuel assembly, the spacer grids 700 and the end plates 300 and 400 are integrated into a single structure using a plurality of guide tubes 500. The guide tubes 500 also form a plurality of passages for receiving a variety of monitoring tubes used for measuring the operational conditions of the reactor.
As shown in FIG. 9, each of the spacer grids 700 are produced by interlacing a plurality of thin metal inner straps at right angles to form an egg-crate pattern, and welding the interlaced straps at their intersections prior to encircling the periphery of the grid with four perimeter straps. A plurality of mixing vanes are provided on the upper portion of each spacer grid 700 for bending the axial flow of coolant to a transverse flow. The spacer grid 700, fabricated by the interlaced inner straps, defines a plurality of four-walled cells for receiving and holding the fuel rods 200 therein, as shown in FIG. 10. In each of the cells, a plurality of grid springs and a plurality of strong dimples are formed on the inner straps such that the springs and dimples face each other. The springs and dimples support the fuel rods 200 in the spacer grids 700.
In the fuel assembly, the fuel rods 200 are axially set in the cells of the spacer grids 700 such that four fuel rods 200 inside four adjacent cells of each spacer grid form a coolant channel, causing the coolant to flow axially, i.e., along the channel. However, the fuel rods of a nuclear fuel assembly typically have different thermal outputs due to an imbalance in the neutron flux distribution, and so the coolant flowing through some coolant channels surrounded by fuel rods having high thermal outputs is highly increased in the temperature in comparison with the coolant flowing through other coolant channels surrounded by fuel rods having low thermal outputs. The thermal outputs of the fuel rods positioned around the high temperature channels are also increased, and so the coolant flowing in said channels is boiled prior to the coolant flowing in the low temperature channels, and forms bubbles on the external surfaces of the fuel rods. Such bubbles are joined together as time goes by, thus forming a bubble layer on the external surface of each fuel rod. The bubble layers prevent heat from transferring from the fuel rods to the coolant, and so the heat transfer condition of the fuel rods may reach so called a critical heat flux condition which increases the temperature of the fuel rods and overheats the fuel rods.
Such an excessive increase in the temperature of the fuel rods causes partial thermal stress on the claddings of the fuel rods, thus reducing the mechanical performance of the fuel rods. When the temperature of the fuel rods is further increased, the temperatures of the core and cladding of each fuel rod may reach their melting points.
The mixing vanes, provided on the upper portion of each spacer grid, bend the axial flow of coolant to a cross flow or a swirl flow by the shapes. The transverse flow of coolant formed by the mixing vanes during the redirection of the axial coolant flow to the cross flow or the swirl flow somewhat relieves the imbalance in the temperature distribution between the coolant channels. In such a case, the increased turbulent flow energy of coolant disturbs the thermal boundary layers of coolant on the external surfaces of the fuel rods, and detaches the bubbles formed on the external surfaces of the fuel rods, thus promoting the heat transfer efficiency on the external surfaces of the fuel rods.
The operation of nuclear reactors has been controlled such that no critical heat flux is generated in the coolant channels of the nuclear fuel assemblies. In order to allow a nuclear fuel assembly to generate power at high output without forming any critical heat flux, the fuel assembly may be controlled such that it has a uniform coolant temperature distribution and prevents the fuel rods from overheating partially.
As described above, the objective of the mixing vanes of the spacer grids is to improve the thermal mixing effect of coolant and thereby improve the thermal efficiency of a fuel assembly. However, the mixing vanes may be undesirably bent or deformed when they are unexpectedly impacted by the fuel rods during an insertion of the fuel rods into a fuel assembly. Such bent or deformed mixing vanes scratch and damage the fuel rod surface due to the contact with the fuel rods during an insertion of the fuel rods into the cells. The mixing vanes also sometimes increase the pressure loss in the spacer grids, and thereby increase the mechanical energy in a nuclear reactor system.
Representative examples of conventional mixing vanes of the spacer grids for nuclear fuel assemblies are described in U.S. Pat. Nos. 4,692,302, 5,299,245, and 5,440,599.
In U.S. Pat. No. 4,692,302 (Inventors: Edmund E. Demario et al., Applicant: Westinghouse Co. Ltd.), two mixing vanes are formed either side of intersection of the inner straps of a spacer grid, such that the two vanes are oriented in opposite directions and guide the axial flow of coolant along the central axis of each coolant channel to the gaps between fuel rods. However, the spacer grid having such mixing vanes is apt to causes a large hydraulic pressure loss while bending the high speed axial flow of coolant along the central axis of each coolant channel to a cross flow.
In U.S. Pat. No. 5,440,599 (Inventors: Thomas Rodack et al., Applicant: Combustion Engineering Co. Ltd.), a mixing vane is positioned at a triangular support provided at the center of each coolant channel. The mixing vane is deflected such that it guides the axial flow of coolant from the center of the coolant channel to the gaps between the fuel rods. However, this mixing vane prone to reduce the cooling efficiency by the reason that the transverse flow of coolant formed by the mixing vanes comes into collision with the axial flow of coolant along the central axis of the coolant channel, thus being disturbed by the axial flow.
In U.S. Pat. No. 5,299,245 (Inventors: Michael E. Aldrich et al., Applicant: BandW Fuel Co. Ltd.), four mixing vanes are formed within each coolant channel at the inner straps around a welded tap provided on either side of intersection of the inner straps. The mixing vanes guide the axial flow of coolant from the center of the coolant channel to the gaps between the fuel rods. However, the spacer grid having such mixing vanes is liable to increase pressure loss by the reason that the four mixing vanes within each coolant channel reduce the opening ratio of the coolant channel in comparison with the other conventional spacer grid having two mixing vanes within each coolant channel.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the conventional art.
It is an objective of the present invention to provide a spacer grid for nuclear fuel assemblies, which is integrated with double deflected vanes at its upper portion for bending the axial flow of coolant into a swirl flow around the fuel rods in the fuel assembly, thus improving the coolant mixing efficiency and more effectively cooling the fuel rods, the vanes of which are also sufficiently wide at their base portions as to prevent an unexpected bending thereof due to their contact with fuel rods during an insertion of the fuel rods into the cells, thus improving the mechanical properties of the spacer grid; and the vanes of which also make a smooth variation in the cross-sectional area of the coolant channel at the outlet of the spacer grid, thus preferably reducing pressure loss at upper portion of spacer grid and thereby improving the hydraulic efficiency of the spacer grid during an operation of the fuel assembly.
In order to accomplish the above objectives, the present invention provides an improvement over the prior art wherein a spacer grid for use in a nuclear fuel assembly has double-deflected vanes that guide an axial flow of coolant around fuel rods and thereby generate swirl flow. The vanes each have a double bend projecting upwardly from first inner straps and projecting toward one fuel rod. The vanes are sufficiently wide at their bases to prevent inadvertent deformation due to contact with fuel rods during an insertion of fuel rods into the cells. The vanes also make a smooth variation in the cross-sectional area of the coolant channel at the outlet of the spacer grid, thus reducing a loss of pressure during reactor operation.